1. Field of Invention
The present invention relates to a method and apparatus for controlling the operation of a pump, such as a centrifugal pump.
2. Description of Related Art
Many known Variable Frequency Drive (VFD) systems create accurate mathematical models of the motors being driven in order to provide precise control over speed and torque, which are used for controlling the operation of pumps. Such known methods and devices include the following:
U.S. Pat. No. 6,591,697 discloses a pump regulating technique based on a relationship of torque and speed versus the pump flow rate and the ability to regulate the pump flow using a Variable Frequency Drive (VFD) to adjust the centrifugal pump speed. However, this technique does not include logic that would provide for protection against undesirable operating conditions, such as a dry run condition, a minimum flow condition, a runout condition, or some combination thereof. Instead, this technique merely utilizes calibrated speed versus torque curves which are application specific to obtain flow thereby reducing flexibility during field setup.
U.S. Pat. No. 6,464,464 sets forth a control and pump protection algorithm which uses a VFD and auxiliary instrumentation to regulate flow, pressure or speed of a centrifugal pump, while other VFD systems utilize flow or pressure switches to identify undesired operating conditions. However, the use of additional process flow switches and other auxiliary instrumentation adds cost and complexity to the drive system, a potential failure point, and unnecessary cost.
U.S. Pat. Nos. 5,930,092 and 5,754,421 disclose pump protection techniques based on an observation of the motor amperage draw and speed and then a correlation of the resulting power reading to various operating conditions (e.g. dry running, closing valves). However, this technique is suitable only for constant speed applications and fails to provide control differentiation for various conditions; protective settings result in only “tripping” or shutting off of the motor.
Another known pump control technique is based on a VFD having parameters that allow maximum and minimum torque values to be configured to prevent the load driver (motor) from operating outside of these parameters. However, this drive technique does not provide logic for interpreting different undesirable operating conditions, nor does it allow for scaling of centrifugal loads, such as pumps or take into account mechanical losses in small pumps at reduced speed.
Other known ways for controlling the operation of pumps include the following: U.S. Pat. No. 4,470,092 discloses a motor protector that trips a motor based on a comparison of one or more sensed trip point parameters and programmed trip point parameters. U.S. Pat. No. 4,827,197 discloses a pump with overspeed protection that adjusts the pump speed based on sensed tachometer and current values, in which the torque is computed based on the sensed current value, an angular acceleration is computed based on the sensed tachometer value, inertia is computed based on the computed torque and angular acceleration, and a table lookup is used to provide a maximum speed of rotation.
U.S. Pat. No. 5,726,881 discloses a pump with overspeed protection that adjusts the pump speed based on two sensed rotational speeds detected by sensors. Similarly, see also U.S. Pat. No. 5,649,893 that discloses a pump with series-implemented protection means. U.S. Pat. No. 5,736,823 discloses a blower and motor combination with constant air flow control that adjusts torque of the motor based on sensed motor speed and current from sensor and flow rate inputs from flow rate input devices, in which speed, torque, pressure and air flow characteristics of the blower are used in making the torque calculation. U.S. Pat. No. 5,742,522 discloses a pump having a digital torque estimator that is used to detect load changes based on sensed current and voltage values with sensors. U.S. Pat. No. 5,917,688 discloses a pump with overspeed protection that adjusts the pump speed based on two sensed rotor and motor speed values detected by sensors. U.S. Pat. No. 6,501,629 discloses a motor with a controlled power line that adjusts the motor power based on sensed motor current and voltage values detected by sensors, in which a measured input power is compared to an input power limited range and the power is disconnected based on this comparison. U.S. Pat. No. 6,679,820 discloses a method for limiting the operational speed of a motor based on a collective evaluation using a method involving rotor and torque tables and including a step of determining an actual ratio of change in acceleration and difference in drag torque speed terms of a rotor in relation to a predetermined range of an expected ratio of change.
The above devices and techniques do not include logic that differentiates undesirable operating conditions to control the pump appropriately for each condition and there is a need in the prior art for controlling the operation of a pump that differentiates between undesirable operating conditions. In some cases auxiliary instrumentation and controls are required.